The present invention relates to improving images produced by electrostatographic, electrophotographic or ionographic printers and reprographic copiers. More particularly, methods and apparatus are disclosed for solving the image problems of edge delineation and edge placement for shapes in an image. Such edge delineation and placement problems manifest as phenomena often referred to as line shrinkage, halo and white gap artifacts. These artifacts are also sometimes referred to as “slow toner”.
Heretofore a number of patents have disclosed various approaches to the manipulation and enhancement of the edges of image shapes. The primary focus of the various approaches has been upon the line and edge smoothing of “jaggies” and other artifacts generated by digitization. A summary follows.
In U.S. Pat. No. 4,847,641 to Tung, print enhancement circuitry to enhance the printed image produced by a laser beam printer is interposed between the character generator circuits and the laser drive circuits to modify the laser drive signals provided by the character generator circuits. Bit data representing successive lines of the bit map for a desired image are stored in a first-in first-out (FIFO) buffer. The bit pattern sample window having a central cell (bit) and a selected (arbitrary) number of neighboring bits is compared to a number of matching bit patterns or templates, each of which is associated with an error element or cell. When a logic matching network detects a match, a modification signal associated with a unique compensation cell (bit) is generated. The sample window central bit is then replaced (modified) with the unique compensation bit required by the matching template. In this manner, all bits in a desired bit map, or set of bit maps, are examined and their corresponding laser drive signals modified to compensate for the errors associated with the matched templates in a piece-wise manner.
U.S. Pat. No. 4,544,264 to Bassetti et al. discloses an electrophotographic printing machine with circuits to enhance the printing of fine lines, such as lines of a single picture element (pel) in width. Provision is made for broadening such lines in one dimension by adding small “black” areas to each edge of the fine line in order to broaden it. In a second dimension, perpendicular to the first dimension, lines are broadened by placing gray pels next to black pels. The disclosure also discusses specific cases in which it may be considered desirable to inhibit the enhancement signals.
U.S. Pat. No. 5,029,108 to Lung discloses an edge enhancement method and apparatus for dot matrix devices wherein a group of gradient mask matrices are applied to a “current matrix”, wherein a target pixel is surrounded by neighboring pixels, to determine if the target pixel is at a location where a change of brightness occurs. From this matrix operation, a conclusion is derived as to the existence or non-existence of an edge and the direction of the brightness change. The current matrix and a predetermined number of previously evaluated and yet to be evaluated pixels are then compared to a set of reference bit patterns which depict possible segment changes to be corrected. If the result indicates that the target pixel is on an edge of a changing edge segment, a corresponding code will be generated to modify the target pixel to enhance the smoothness of a segment transition. In the case of an electrophotographic printing machine, the specific code will change either the location or the size of the target pixel; whereas in the case of a monochrome screen display, the specific code will change the intensity of the target pixel.
In U.S. Pat. No. 4,544,922 to Watanabe et al., a desired character mainly composed of standard width dots selected from a matrix of orthogonally disposed rows and columns is displayed on a screen during scanning of the screen in horizontal and vertical directions. The display is smoothed by a circuit responsive to data stored in a memory. The smoothing involves the selected addition or removal, to or from particular portions of the character, of a small dot having a width one-third of the standard dot width.
U.S. Pat. No. 4,625,222 to Bassetti et al. discloses print enhancement circuits for an electrophotographic printing machine are placed between the character generator and the printhead to modify drive signals for the printhead. Modifications include smoothing the digitized edges of slanted lines; broadening single pel width lines in the direction perpendicular to the scan direction as well as in the direction parallel to scan. Inhibiting circuits are provided to prevent passage of enhancement signals under certain conditions. Generally, leading and trailing edge gray signals are provided next to all black data in a direction parallel to scan while expanded black signals are provided for the single pel data in a direction perpendicular to scan by adding to the black signal on both its leading and trailing edges. When a single picture element (pel) area contains two added black signals, both are passed; when a single pel area contains one added black signal and one gray signal, both are passed; when a single pel area contains two gray signals, only the leading gray signal is passed; and when a single pel area contains two added black signals and a gray signal, only the gray signal is passed.
The U.S. Pat. No. 5,479,175 to Cianciosi et al. is a an apparatus for enhancing the output along edges of discharged area developed regions in a tri-level imaging system employing a pulse width and position modulated signal ROS for exposure. The invention enables the identification and selective alteration of video data used to drive the ROS so as to extend the developed regions by a selected amount and eliminate digitization artifacts present in the image to be printed. The extension of the discharged area developed regions is accomplished by extending the width of, or adding separate, exposure pulses in adjacent areas to enable development within a portion of those regions.
In U.S. Pat. No. 5,193,008 to Frazier et al., the output of a conventional laser printer having a resolution of 300×300 dots per inch (dpi), and a predetermined threshold level for forming image dots, is enhanced by selectively providing interleaved image dots between the normal scan lines of the laser printer. Such interleaved image dots between scan lines may be achieved by appropriately energizing the two pixels directly above and directly below that desired interleaved dot, with the energizations at one or both pixels being selectively below the threshold level for producing a dot on the scan line, but with the combined energization at the desired interleaved point being above the threshold level to produce the desired interleaved dot. An input 600×600 bit map may be stored in a random access memory, and three vertically aligned bits from one main scan line and adjacent 600 dpi lines above and below are drawn from the RAM and are supplied to a logic and video output circuit which produces variable pulse width modulated pulses to the laser printer to produce the enhanced image.
U.S. Pat. No. 3,784,397 discloses a method for forming images by providing an electrostatographic imaging member bearing an electrostatic latent image on a recording surface. Then positioning the recording surface spaced from and facing a development electrode. This is followed with contacting the recording surface with toner particles whereby at least a portion of the toner particles deposit on the recording surface to form at least a partially imaged recording surface. Then maintaining the field strength of the development electrode as weak during the initial period of development and then increasing the field strength of the development electrode during the latter period of development, to form a substantially uniform developed image substantially free of streak, halo, edge effect, and background deposits.
In conventional xerography, electrostatic latent images are formed on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor is advanced to a development station where toner is attracted to the areas not discharged on the exposed charge retentive surface. In the development station there is a developer housing which is typically an inch or two (or more) long in the process direction. However, development doesn't occur throughout this length, but only over a restricted region. Typically this is a region where the photoreceptor belt or drum comes very close to the developer agent, whether it is a magnetic brush roll (which is roughly circular in cross section), or a donor roll. This area of closest approach or actual contact is called the nip. It is typically a few millimeters or so long (in the process direction). This is the region where all of the toner is actually transferred to the photoreceptor. Typically there is an air gap between the source of the toner and the photoreceptor.
A toner cloud is used to span the air gap between the source of toner and the photoreceptor surface. A method and apparatus for producing such a cloud of toner in an air gap between a toner donor and the photoreceptor is described in detail in U.S. Pat. No. 4,868,600, which is herein incorporated in its entirety by reference. Toner is detached from the toner donor and a powder cloud is generated by AC electric fields supplied by self-spaced electrode structures positioned within the development nip between the toner donor and the photoreceptor. The electrode structure is placed in close proximity to the toner donor within the gap between the toner donor and image receiver or photoreceptor. The toner cloud is used to span the gap between the source of toner and the photoreceptor surface as the photoreceptor passes through the development station. As through-put requirements drive the passing of the exposed photoreceptor surface though development station at ever greater speeds, this toner cloud is directed and modulated by the field charge pattern that results on the photoreceptor after exposure. Thus it is the speed and the presence of a gap that allows fringe fields in the latent image to strongly influence the deposition of toner. This is exacerbated in scavengeless systems where a cleaning field is utilized to repel toner from the photoreceptor.
The observed result of this toner cloud modulation is a propensity for depositing large amounts of toner where there is uninterrupted expanse of charged area (such as toward the middle of large image shapes), and to starve toner from locations where there is a strong or sudden change in charge (as found with narrow lines or shapes and on the edges of larger shapes). The result for thin lines and narrow shapes is line shrinkage. The effect on large shapes causes them to exhibit a defect called halo, which manifests itself most clearly at the interfaces of solid colors. Halo in color systems appears as white lines at interfaces which should otherwise be a perfect match between two colors. This defect is also observable in single color images as an edge distortion or displacement and line shrinkage dependent on the size of the printed object. Line shrinkage of course leads to poor line and text quality due to an erosion or shrinkage of the line edges and corners.
Therefore, there exists a need for techniques which will solve these halo and slow toner effects. Further, there exists a demand for increasing the throughput of printing and digital imaging systems without incurring or exacerbating these problems. Thus, it would be desirable to provide a means for satisfying such needs or demands by solving the aforesaid and other deficiencies and disadvantages.